On-fiber tunable bragg gratings for DWDM applications

ABSTRACT

A method and apparatus for tunable on-fiber Bragg gratings for DWDM and other applications on a small section of the core of single mode communication of an optical fiber. The method comprises etching most of the cladding on a small section of the fiber; coating the etched portion with a metallic electrode material and then with a layer of an electrooptic material; coating the electrooptic material with a photoresist; producing the Bragg grating pattern using a holographic process or on-axis interferometry; dissolving the non-exposed photoresist,; etching the grating pattern into the electrooptic material, and coating the Bragg gratings with a metallic material constructing the outer electrode. The presence of an electric signal on the electrodes will change the optical properties of the electrooptic material, as well as the diffraction/reflection properties of the Bragg gratings.

FIELD OF THE INVENTION

This invention relates to fiber optics and more particularly to anapparatus and method for constructing tunable Bragg gratings on anoptical fiber core for uses as optical multiplexer, de-multiplexer,coupler, or filter. The invention is used for dense wavelength divisionmultiplexing, (DWDM), optical filters, or other devices.

BACKGROUND OF THE INVENTION

This is an improvement of U.S. Pat. No. 4,842,405, where I am able toprovide a tunable on-fiber Bragg gratings for telecommunication andother applications. The Bragg gratings are used in telecommunicationlinks as a means for multiplexing, demultiplexing, coupling, orfiltering optical signals at the Bragg wavelength. It is desirable toimpart Bragg grating patterns on the surface of optical fibers. Themethods available of producing Bragg gratings on the surface of opticalfibers are essentially limited to producing the gratings on a flattenedportion of the fiber and the grating thus produced covers only a smallpercent of the surface of the optical fiber core resulting in apolarization dependent structure. The only exception is the U.S. Pat.No. 4,842,405 which provides a method for constructing polarizationindependent Bragg gratings onto the entire cylindrical surface of theoptical fiber, however, the Bragg gratings are not tunable.

It would be of great advantage for the art if tunable multiplexers,demultiplexers, couplers and filters could be built by forming tunableBragg gratings on the entire cylindrical surface of the optical fibercore in a small section of the fiber is disclosed.

Another advantage would be to impose tunable gratings on the entiresurface of the optical fiber as a polarization independent apparatus,the application of the device in telecommunication systems is vastlyincreased.

Other advantages will appear hereinafter.

SUMMARY OF THE INVENTION

It has now been discovered that the above and other advantages of thepresent invention may be achieved in the following manner. Thisinvention is directed to an apparatus and method for building tunablemultiplexers, demultiplexers, couplers and filters by producing atunable Bragg grating structure on a section of an optical fiber whereinthe grating pattern is written into an electrooptic material coated ontothe fiber core to cover the entire cylindrical surface of the modifiedsection.

The method for producing the tunable gratings on the surface of thefiber core requires etching the cladding layer on a small section of thefiber to reduce the thickness of the cladding layer to a very thinlayer, then, coating the modified section of the optical fiber with athin layer of metallic material. This coating forms the inner metallicelectrode. Then, the inner electrode is coated with a layer of anelectrooptic material. Various methods can be used for writing thegratings into the electrooptic material, using a photoresisit coating.The photoresist coated on the electrooptic material is exposed to aninterference pattern produced from a laser light source. The exposedphotoresist is developed to produce therein a Bragg grating pattern onthe modified section of the optical fiber. The produced Bragg gratingpattern is reproduced into the surface of the electrooptic material inthe modified section. Then, a layer of metallic material is coated onthe top of the electrooptic material. This coating forms the outermetallic electrode. To facilitate the exchange of optical signal betweenthis modified fiber and other communication links, this fiber isconnected to a bi-directional coupler or it is mounted on a photonicschip. Packaging of the device requires coating the modified section withan external cladding.

An alternative structure of the apparatus is achieved by writing thegratings into the cladding layer next to the fiber core before applyingthe eletrooptic material. This requires etching the cladding in a smallsection of the fiber to reduce the thickness of the cladding layer to afew micrometers. Then, this modified section of the optical fiber iscoated with a photoresist, and exposing the photoresist to aninterference pattern produced from a laser light source. The exposedphotoresist is developed to produce therein a Bragg grating pattern onthe modified section of the optical fiber, followed by etching the Bragggrating pattern into the thin cladding layer next to the core, in themodified section. A multilayer coating is applied on the top of theBragg gratings wherein an inner metallic electrode layer, anelectrooptic material layer, and an outer metallic electrode layer arecoated on the top of the Bragg gratings. All these coatings are applieduniformly in a cylindrical shape for a polarization independent tunableBragg gratings structure. Packaging of the device is performed in thesame way as above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention described herein is shown by reference to the drawings,although it is to be understood that the drawings are referred to onlyfor purposes of illustration and example, and the scope of the inventionis not limited thereto. For a more complete understanding of theinvention, reference is hereby made to the drawings, in which:

FIG. 1 is a pictorial view of a section of a typical optical fiber 1constructed of a core 2 and cladding 3;

FIG. 2 is a side elevational view partially broken away of the opticalfiber of FIG. 1 having the cladding portion of the fiber substantiallyreduced in thickness and the fiber is coated with a layer of metallicmaterial 5, then with a layer of an electrooptic material 7;

FIG. 3 is a side-elevational view showing the optical fiber of FIG. 2having a Bragg grating pattern 4 covering the entire surface of saidmodified section;

FIG. 4 is a side-elevational view showing the modified section of FIG. 3is coated with a thin layer of metallic material 6, then a cladding andpackaging layer 8 is applied on the top of the metallic material 6. Acontrol signal 9 is applied to the metallic electrodes for tuning theBragg wavelength;

FIG. 5 is a pictorial view of the on-fiber device shown in FIG. 4;

FIG. 6 is a side-elevational view showing the modified section of thefiber is mounted on a photonics chip 10;

FIG. 7 is a cross-sectional view (A-A) taken at the center of themodified section shown in (a) FIG. 4 and (b) FIG. 6;

FIG. 8 is a side-elevational view of the optical fiber of FIG. 1 havingthe cladding portion of the fiber reduced in thickness to a fewmicrometers;

FIG. 9 is a side-elevational view showing the optical fiber 1 afterwriting the Bragg grating pattern 4 into the modified section of FIG. 8;

FIG. 10 is a side elevational view showing another embodiment of theinvention, said showing the gratings of FIG. 9 are covered with fourlayers of materials; a first metallic layer 5, an electrooptic material7, a second metallic layer 6, and a cladding/packaging material 8;

FIG. 11 is a side-elevational view showing the modified section of thefiber is mounted on a photonics chip 10; and

FIG. 12 is a cross-sectional view (B-B) taken at the center of themodified section in (a) FIG. 10 and (b) FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a typical optical fiber 1 with a core section 2 surroundedby a cladding section 3. This invention provides a new class of opticalfiber multiplexers, demultiplexers, couplers, and filters. The methodsto produce these devices are also part of this invention. The method toproduce the optical fiber apparatus as shown in FIG. 4 describes a smallmodified section wherein the cladding has been removed from core 2 and athin metallic material 5 is coated on the fiber and core to form a firstelectrode. Then a layer of an electrooptic material 7 is coated on thetop of the portion of the metallic material or electrode 5 that is onthe core 2. A Bragg grating pattern 4 is written into the electroopticmaterial 7 covering the entire surface of this modified section.

FIG. 4 further shows the invented apparatus wherein two layer ofmaterial 6 and 8 are coated on the top of the grating pattern 4generated into the electrooptic material 7. The first layer is a thincoating of metallic material forming the second (or outer) electrode 6,and the second layer is a packaging material 8 used for support andprotection of the apparatus. The pictorial view of the apparatus shownin FIG. 4 is shown in FIG. 5 wherein the inner (or first) electrode 5and the outer (or second) electrode 6 are used to apply external controlsignal 9 for tuning of the Bragg wavelength.

The first step of the process of this invention to produce the apparatusis to reduce the thickness of the cladding material in a section of theoptical fiber to a very thin layer as shown in FIG. 2. This can be doneby chemically etching or other etching means on the optical fiber 1.Reproducible results can be obtained by determining the rate of etchingof the etchant and then carefully monitoring the etching time.

The next step is to coat the modified section with a thin layer ofmetallic material constructing the first electrode 5 shown in FIG. 2.The coating process can be performed using any of the deposition orcoating techniques including plasma deposition, chemical deposition,liquid phase deposition, or any other techniques. Then, a layer of anelectrooptic material 7 is coated on the top of the first (or inner)electrode 5 as shown in FIG. 2.

The next step is to apply a photoresist and preferably a liquidphotoresist uniformly to the entire surface of the electrooptic material7 where the grating 4 is to be applied. The length of the section towhich the grating 4 is to be applied is generally ranging from a fewhundred of micrometers to a few millimeters in length. A holographic orinterference method is used to create an interference pattern at thearea of the photoresist coating on the optical fiber. The interferencepattern exposes the photoresist to duplicate the interference pattern ondevelopment of the exposed photoresist by conventionally known means.

The process for writing the Bragg gratings uses the holographic methodor the interference pattern method, which can be produced by reflectingthe laser beam back upon itself to expose the modified section coatedwith a photoresist followed by development of the exposed photoresistand then etching of the interference pattern produced in the developedphotoresist to produce a Bragg grating pattern into the electroopticmaterial in the modified section of the optical fiber.

The interference pattern is reproduced in the developed photoresist inthe form of a sinusoidal, triangular, or other pattern having peaks andvalleys which is preferred as Bragg gratings.

FIG. 3 shows the grating pattern 4 etched into the electrooptic material7 formed by subjecting the developed pattern to different types ofetching. Etching of the Bragg grating pattern 4 into the optical fiberwill depend upon the composition of the electrooptic material. Chemicaletching, plasma etching and ion milling which are well known in the artcan be employed.

The exposure and development of the photoresist can be done individuallyor it can be done simultaneously as taught by El-Sherif, U.S. Pat. No.4,842,405. The process is not limited to operation on a single opticalfiber. Many fibers can be placed in the path of the laser beam and eachexposed at the same time. The period of the grating can be controlled inthe simultaneous exposure and development method by changing the indexof refraction of the developer. Tuning the Bragg wavelength of theproduced gratings is achieved by applying external control signal 9 tothe electrooptic material 7. The advantages obtained by my processresult from the uniformity of the electromagnetic field applied to theBragg gratings 4. Uniform field exposure on the entire cylindricalsurface of the optical fiber is obtained, which eliminates distortionsin the gratings.

FIG. 6 shows an alternative method of packaging the apparatus,accomplished by mounting the apparatus on a photonics chip 10 for properexchange of optical signals during multiplexing or demultiplexing.

FIGS. 8-12 show another embodiment of the invented apparatus wherein thecladding is reduced to a thin layer as shown in FIG. 8. Then the gratingpattern 4 is written into the thin cladding layer in the modifiedsection as in FIG. 9. The process for constructing the gratings issimilar to the one set forth above using a photoresist material.

An interference pattern exposes the photoresist to duplicate theinterference pattern on development of the exposed photoresist byconventionally known means. The diffraction pattern reproduced in thedeveloped photoresist in the form of a sinusoidal, triangular, or otherpattern having peaks and valleys which is preferred.

FIG. 9 shows the grating 4 etched into the cladding 3 formed bysubjecting the developed pattern to chemical etching. In FIG. 9, theetching extends into the cladding which has a thickness of about 1 to 3microns. Another embodiment of the invention is preferred where theetching extends to the interface of the core and cladding. The exposureand development of the photoresist can be done individually or it can bedone simultaneously.

Etching of the diffraction pattern into the optical fiber will dependupon the composition of the optical fiber. Chemical etching, plasmaetching and ion milling which are well known in the art can be employed.For example, an optical fiber cladding composed of glass can be etchedwith either ion milling or with hydrofluoric acid.

The process is not limited to operation on a single optical fiber. Manyfibers can be placed in the path of the laser beam and each exposed atthe same time.

The period of the grating can be changed in the simultaneous exposureand development method by changing the index of refraction of thedeveloper.

FIG. 10 shows an embodiment of the invented apparatus wherein threelayers of material are on the top of the grating pattern 4 generatedinto the thin layer of the cladding 3 left in the modified section. Thefirst layer is a thin metallic layer constructing the first (or inner)electrode 5. The second layer is an electrooptic material 7 coated ontop of the first electrode 5. The third layer is a thin metallicmaterial forming the second (or outer) electrode 6. A fourth layer canbe coated on the top of the second electrode 6 constructing a claddingand packaging layer 8.

FIG. 11 shows an alternative method of packaging the apparatus, which isby mounting the modified section of the fiber on a photonics chip 10 forproper exchange of optical signals during multiplexing ordemultiplexing.

It should be noted that in both structures shown in FIGS. 4 and 10, theinner and outer electrodes are connected to a signal means or powersupply 9. The power supply is used to modulate the optical properties ofthe electrooptic materials which in turn modulates the Bragg gratingwavelength. This control signal 9 is used for tuning the Braggwavelength.

While particular embodiments of the present invention have beenillustrated and described, it is not intended to limit the invention toany specific embodiment except as defined by the following claims.

1. A method of producing a tunable Bragg grating pattern in an optical fiber having a cladding layer and a core layer, comprising the steps of: etching a section of said cladding layer to produce modified section having a thin cladding layer; coating said modified section with a metallic material to form an inner electrode; coating said inner electrode with an electrooptic material; coating said electrooptic material with a photoresist and forming a Bragg grating pattern within said photoresist by UV exposure and development; copying said Bragg grating pattern onto the electrooptic material by etching said photoresist, and thereafter etching said Bragg grating pattern into said electrooptic material; forming an outer electrode on said electroptic material; and enclosing said section with outer packaging material.
 2. The method of claim 1, wherein said photoresist is a liquid photoresist and said photoresist material is exposed to an interference pattern formed by reflecting a beam of a laser on a reflecting surface placed perpendicular to the direction of the optical fiber axis and the direction of the laser beam to thereby expose said photoresist to an on-axis interference pattern, said exposed photoresist thereafter being developed to reproduce said interference pattern on said modified section prior to said etching of said Bragg grating pattern.
 3. The method of claim 2, wherein said photoresist is exposed and developed simultaneously to form said Bragg grating pattern, said exposure being done using holographic or interference techniques.
 4. The method of claim 1, wherein said optical fiber is selected from single mode and multimode optical fibers.
 5. The method of claim 1, wherein said modified section is from about 100-200 micrometers to about 2-3 millimeters.
 6. The method of claim 1, wherein said photoresist is exposed to a Chrip interference pattern producing Chirp Bragg gratings in said photoresist layer.
 7. The method of claim 1, which further includes means for applying an external electric signal to said inner and outer electrodes to permit tuning the wavelength of said Bragg grating in said modified section of said optical fiber.
 8. The method of claim 1, which further includes the step of writing a polarization independent Bragg grating pattern into a different section of the fiber having a partially etched cladding, said Bragg grating being formed before the modified section is coated with said inner electrode, said electooptic material, said outer electrode, and said outer packaging material.
 9. The method of claim 8, wherein cladding is reduced in said different section of said optical fiber to a thickness of about 2-3 microns.
 10. Apparatus having a tunable Bragg grating pattern in an optical fiber having a cladding layer having a Bragg grating pattern therein and a core layer, in which the index of refraction of the cladding is changeable, said apparatus being formed by: etching a section of said cladding layer to produce modified section having a thin cladding layer; coating said modified section with a metallic material to form an inner electrode; coating said inner electrode with an electrooptic material; coating said electrooptic material with a photoresist and forming a Bragg grating pattern within said photoresist by UV exposure and development; copying said Bragg grating pattern onto said electooptic material by etching said photoresist, and thereafter etching said Bragg grating pattern into said electrooptic material; forming an outer electrode on said electrooptic material; and enclosing said section with outer packaging material.
 11. The apparatus of claim 10, wherein said photoresist is a liquid photoresist and said photoresist material is exposed to an interference pattern formed by reflecting a beam of a laser on a reflecting surface placed perpendicular to the direction of the optical fiber axis and the direction of the laser beam to thereby expose said photoresist to an on-axis interference pattern, said exposed photoresist thereafter being developed to reproduce said interference pattern on said modified section prior to said etching of said Bragg grating pattern.
 12. The apparatus of claim 10, wherein said Bragg gratings are constructed to diffract the light propagating within said fiber core at the Bragg wavelength and not diffract all other wavelengths to function as a wavelength division de-multiplexer.
 13. The apparatus of claim 12, wherein said Bragg gratings are constructed for coupling an incident light into said fiber core at the Bragg wavelength to function as a wavelength division multiplexer.
 14. The apparatus of claim 13, wherein said modified section of said optical fiber is mounted on a photonics chip for said signal multiplexing or demultiplexing.
 15. The apparatus of claim 10, wherein said Bragg gratings are constructed to reflect light in said fiber core at the Bragg wavelength to function as an optical filter.
 16. The apparatus of claim 10, wherein said fiber is connected to a bi-directional coupler for said signals coupling and filters applications.
 17. The apparatus of claim 10, wherein a photoresist is exposed to a Chrip interference pattern producing Chip Bragg gratings in said photoresist layer, wherein said Chip Bragg gratings function as a device selected from the group consisting of multiplexer, de-multiplexer, coupler and optical filter,
 18. The apparatus of claim 10, which further includes means for applying an external electric signal to said inner and outer electrodes to permit tuning the wavelength of said Bragg grating in said modified section of said optical fiber.
 19. The apparatus of claim 10, which further includes a polarization independent Bragg grating pattern written into a different section of the fiber having a partially etched cladding, said Bragg grating being formed before the modified section is coated with said inner electrode, said electrooptic material, said outer electrode, and said outer packaging material.
 20. The apparatus of claim 19, wherein cladding is reduced in said different section of said optical fiber to a thickness of about 2-3 microns 